In making finished ophthalmic lenses, particularly prescription lenses for eyeglasses, it is customary to begin with semi-finished lens blanks made from glass or plastic. The blanks have a finished, polished front surface and an unfinished back surface. They are surfaced to a particular prescription by grinding material from the unfinished back surface followed by fining and polishing so that they acquire the optical refractive properties specified in the prescription. The lenses may then be shaped or edged to fit the spectacle frame selected by the wearer.
It is essential that a lens be positioned accurately and held securely during the surfacing and edging operations. However, edge clamping techniques, such as mounting the lens in a vise or in the jaws of a chuck, are unsuitable for holding the lens because material is removed from both its back surface and edges. Thus, it is necessary that the lens be held by an adhesive means which secures it by the finished surface in the appropriate position in the grinding machine. This may be accomplished by "blocking" the lens, e.g., adhering a lens block to the lens by means of a fusible metal alloy or polymeric material.
The alloy or polymeric material is applied in a molten state and subsequently allowed to harden to form a rigid layer of predetermined size and shape that bonds the lens to the lens block. While the alloy conforms well to the front surface of the lens blank, the strength of the bond between the alloy or polymeric material and the lens blank is low. Consequently, primers are needed to obtain adequate bonding between alloy or polymeric material and lens. Primers, however, cause certain problems. For example, they are typically applied from a solution by, for example, brushing or spraying. Consequently the solvent must be allowed to evaporate before the surfacing and edging processes can proceed. This causes inconvenience and delay in processing the lens blanks.
Other techniques of blocking lens blanks have also been tried. Thus, sticky substances, such as pitch or wax, double-sided sticky constructions such as pads or foams coated on each face with adhesive, and epoxy adhesives have been used. These means also have not proven entirely satisfactory. For example, the residue left by pitch, wax, and adhesives require extensive clean-up of both lens and block. This causes delay and added expense in the processing operations. Additionally, these techniques provide less rigid mounting means than do the alloy or polymeric material bonded blocks. Consequently, it is more difficult to assure that the lens will be properly positioned throughout the entire surfacing and edging processes. Additionally, pads or foams are typically opaque so that it is difficult to properly align the lens in the surfacing or edging apparatus. Additionally, the pads and foams are not satisfactorily conformable to the complex curvature of a lens face. Thus wrinkles, folds, air bubbles and other discontinuities between the lens blank and the pad or form are present when they are employed.
A conformable, multi-layered tape for bonding fusible metal alloy to ophthalmic lens blanks is described in U.S. Pat. No. 4,287,013 (Ronning). Unfortunately, the tapes described in this reference generally require fairly complicated processing methods and, as a result, are relatively expensive to produce. In addition, presently available surface protection tapes require a separate liner material to prevent adhesion build-up between the layers of the wound roll of tape. This additional liner material adds undesirably to the total cost of the roll of tape. There is a need for a lens blank surface protection tape that exceeds the performance of the prior tapes and is more cost efficient.